This application is based on and claims the benefit of German Patent Application No. 10242254.0 filed Sep. 12, 2002, which is incorporated by reference herein.
This invention relates to an electric cable, for connecting mobile electric loads (current utilization devices), having at least two strands, each consisting of insulated conductors stranded together and having insulation consisting of layers arranged one above the other and fixedly bonded together, the inner layer being in contact with the conductor and being softer than the outer layer, with the strands surrounded by a common inner sheathing of insulation material which fills up outer gaps between the strands, and with an electric shield and an outer sheathing of insulation material also being present (as discussed in German Patent Application 100 36 610 A1).
Such cables are to be used as flexible cables for connecting portable devices having a voltage source and/or signal source. Such devices may include, for example, cranes, machine tools and robots. The cables must have a high load capability, must have a uniform bending fatigue strength over a long term and good flexibility over a broad temperature range, e.g., between xe2x88x9240xc2x0 C. and +80xc2x0 C.
With known cables of this type available on the market, the strands are surrounded by a loose inner sheathing of polyvinyl chloride (PVC) or polyurethane (PUR) which functions as filling. The strands are therefore relatively loose and may be damaged easily from the constant bending, back and forth, of the respective cable. With such inner sheathings, they are either not free of halogen and contain lead-based stabilizers (PVC) or they are not flame retardant (PUR), so that subsequent damage may occur in the event of a fire or the cables may quickly lose their functionality.
With the known cable according to the German Patent Application 100 36 610 A1 cited in the introduction, the softer inner layer of the strand insulation consists of a rubber-elastic material containing polypropylene, while the outer layer consists of a material based on polypropylene. The strands therefore have flexural strength even when subjected to repeated bending of the cable. They also have a good non-stick property, which supports their fatigue strength under reversed bending stresses. The inner sheathing, which surrounds the strands and functions essentially as filling, imparts increased stability to the cable if it also fills up the outer gaps between strands. The publication cited does not mention anything about the material of the inner sheathing or the type and arrangement of the electric shield.
The object of the present invention is to improve upon the cable described in the background, in particular with respect to its bending fatigue strength and its twistability combined with simplified fabrication.
This object is achieved according to this invention by providing a separator layer between the strands and the inner sheathing to ensure a relative mobility between the strands and the inner sheathing. The inner sheathing consists of two layers arranged one above the other and fixedly joined together, the inner layer of which, i.e., the layer facing the strands, is softer than the outer layer. The inner sheathing has an approximately circular peripheral surface and lies firmly against the separator, and thereby also indirectly against the strands. The electric shield arranged above the inner sheathing consists of at least one essentially closed tubular metallic layer and a stranded layer or braiding of metallic wires adjacent with the metallic layer, the outer sheathing being positioned above the shield.
This cable has a long-term bending fatigue strength, which is stable over the long run, so that it is particularly suitable for traversing long distances and withstanding frequent changes of direction in bending. Such requirements are frequently met in the case of machine tools. The strands themselves are designed with their special two-layer insulation with a softer layer on the inside for frequent changes in the direction of bending. On the whole, they are immovably secured radially in the inner sheathing, which fixedly surrounds the strands like a type of corset and therefore effectively protects them from the mechanical forces that occur in bending.
The separator layer applied between the strands and the inner sheathing, however, ensures that the inner sheathing is movable relative to the strands, so that the good bending properties of the cable are not impaired. Instead this yields a very good compensating movement of the strands between the upending and compression zones.
The good bending properties of the cable are also supported by the design of the inner sheathing, which consists of two layers of different hardness fixedly joined together, the softer layer of which faces the strands. This structure of the inner sheathing also allows an especially simple fabrication of the cable. As circumstances require, only the outer harder layer of the inner sheathing need be severed. It can then be torn away easily at the separation point without any risk of damage to the strands because the separator layer is provided between them and the inner sheathing.
Regardless of the number of strands, the inner sheathing has an approximately circular peripheral surface. It thus provides a fixed and uniform substrate for the electric shield of the cable and its outer sheathing of insulation material situated above same. The electric shield consisting of at least one metallic layer, which is closed on all sides and a cabled covering/braiding adjacent thereto, is electrically impervious even at higher frequencies. Therefore, no interfering radiation can be emitted by the cable, and the cable itself is effectively protected from foreign fields.